HR MAS NMR coils with magic angle capacitors

ABSTRACT

A method of positioning chip capacitors in the Alderman-Grant and related Half-Turn coils is disclosed that permits substantially improved main field homogeneity in HR MAS and conventional &#34;wideline&#34; NMR coil geometries. Eight capacitors are positioned at the complements of the magic angle with respect to the B1 axis at each end of the coil.

RELATED APPLICATIONS

This application claims priority from pending U.S. ProvisionalApplication Ser. No. 60/041,317, filed on Mar. 20, 1997, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The field of this invention is the measurement of nuclear magneticresonance (NMR and MRI) for the purpose of determining molecular ormicroscopic structure, and, more particularly, a low-inductance rf coiltuning arrangement for improved B₀ homogeneity in Magic Angle Spinning(MAS) NMR.

BACKGROUND OF THE INVENTION

This invention pertains to improving the B₀ homogeneity in highresolution (HR) MAS NMR, especially for liquids and semi-solids at highfields, using low-inductance saddle coils on cylindrical surfacesinclined at 54.7° with respect to B₀. Related NMR coils are described byDoty in U.S. Pat. No. 4,641,098, and numerous coils are reviewed byJames Hyde in `Surface and Other Local Coils for In Vivo Studies`, Vol.7, of The Encyclopedia of NMR, Wiley Press, 1996. A copendingapplication discloses related coils using litz foil, and anothercopending application discloses novel sample cells for HR MAS. Hilldiscloses one method of geometric compensation using oval saddle coilsfor HR NMR in U.S. Pat. No. 4,563,648.

There have been numerous applications of Magic Angle Spinning (MAS) forline narrowing in solid samples for more than two decades. The solidsample is usually contained in a hard ceramic rotor with press-fitturbine caps machined from high-strength high-modulus plastics such aspolyimides--see for example, U.S. Pat. No. 5,508,615 by Doty et al (notethe extensive list of typographical corrections). The coil hastraditionally been a multi-tuned solenoid, as shown by Doty in U.S. Pat.No. 5,424,625, although Cory et al in U.S. Pat. No. 5,539,315 have useda loop-gap resonator in combination with a solenoid. Another copendingapplication discloses thermal buffering and susceptibility compensationto permit the use of transverse coils inside HR MAS solenoids, and thisinvention is directed toward improving B₀ homogeneity of such.

The recent semi-solids MAS applications stem largely from the fact thatspinning a cylindrically symmetric sample at the magic angle averagesits susceptibility effects to zero. Moreover, spinning at the magicangle averages the inhomogeneities produced by static magnetic cylindersaligned with the magic angle to zero. Hence, high resolution may beobtained with magnetically inhomogeneous samples, such as tissues andsemi-solids, and the inhomogeneities produced by the cylindricalportions of the stator, coil, and housing are inconsequential. This isparticularly important for applications with limited samples. While theliterature is replete with attention to compensation of the magnetism ofrf coils for HR NMR, the capacitor magnetism problems have been largelyignored.

Kost et al and the above referenced copending litz-foil applicationdisclose methods of improving the B₁ homogeneity of the conventionalslotted-resonator, the Alderman-Grant half-turn resonator, and otherrelated coils. However, for small samples at very high fields,performance is limited primarily by the poor B₀ homogeneity that comesfrom the proximity of the four chip capacitors traditionally used in theAlderman-Grant and related HT coils. In high-resolution NMR, theseproblems have been largely circumvented by using distributed capacitorswith cylindrical symmetry, but this approach does not work well in HRMAS because of the severe space constraints imposed by the spinner.Magnetic compensation of the chip capacitors is only partiallysuccessful because of the temperature and field dependence of theirmagnetism.

Various circularly polarized birdcage resonators, as disclosed byEdelstein et al in U.S. Pat. No. 4,680,548, in which 8 or morecapacitors are uniformly spaced around the rings at each end, are oftenused for large samples, and they have also been shown to be usable forsamples of the size encountered in HR MAS. However, they are much lessefficient and much more difficult to tune than the coils of the presentinvention for small MAS applications. Compared to single-turn orhalf-turn coils, current concentrations and hence peak conductor edgeheating are greatly reduced in multi-turn transverse (Zens) coils. Also,their high inductance premits the capacitors to be well removed from theregion of critical B₀ homogeneity, but their high inductance causessevere high-voltage breakdown problems for high-power at highfrequencies and limits the frequency at which they may be used.

SUMMARY OF THE INVENTION

A method of positioning chip capacitors in the Alderman-Grant andrelated Half-Turn (HT) coils is discloses that permits substantiallyimproved B₀ homogeneity in HR MAS and conventional "wideline" NMR coilgeometries. The four segmenting and tuning capacitors normallypositioned at opposite ends of the HT coil and aligned with the B₁ axisare replaced by eight capacitors positioned at the complements of themagic angle (54.7°) with respect to the B₁ axis at each end. The twocapacitors may be paralleled at each node by magnetically compensatedarcs. Magic angle capacitor positioning may also be applied to relatedcoils with intersecting loops and to segmented loops in parallel withunsegmented loops.

BRIEF DESCRIPTION OF THE DRAWINGS

1. FIG. 1 illustrates the prior art Split-Half-Turn (SHT) coil.

2. FIG. 2 discloses a method of paralleling magic angle capacitors inthe HT coil.

3. FIG. 3 is a perspective drawing of a completed spinner assembly withmagic angle capacitors.

4. FIG. 4 discloses a method of improving current distribution withmagic angle capacitors.

5. FIG. 5 discloses a preferred method of improving current distributionwith magic angle capacitors.

6. FIG. 6 illustrates the etched-half-turn fixed-frequency (EHTF) coilwith magic angle capacitor mounting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optimized parallel-single-turn segmented saddle coil (similar toFIG. 7 of U.S. Pat. No. 4,641,098) shown laid out flat in FIG. 1, whichwe call the split-half-turn (SHT) coil, has somewhat lower edge-currentdensity than prior art Alderman-Grant-type resonators and improved B₁homogeneity. The SHT coil consists of two paralleled semi-coils, eachsubtending less than 180° of azimuthal arc, on opposite sides of acylindrical coilform. The optimized SHT coil solves voltage breakdownproblems, but other low-inductance transverse resonators result in moreuniform current distributions and hence more reduction in conductor edgeheating.

As important as Q, filling factor h_(F), and B₁ homogeneity are, B₀homogeneity is a more critical problem for many HR MAS applications. Thefour chip capacitors 1, 2, 3, 4 cannot be made strictly non-magnetic.Normally, the electrodes and terminations are 25%Pd-75%Ag, which ishighly paramagnetic. Even with special Au--Cu--Ag terminations, thecapacitors are always found to have unacceptable and unpredictablesusceptibilites, typically in the range of 7 to 12E-6 (SI volumetricunits) at 7 T, 300 K. If the capacitors could be distributed in aclosely spaced full circle around the MAS sample container and if theyall had equal magnetization, the inhomogeneity would largely average tozero under MAS. However, neither condition is easily met--owing todiscrete size limitations, magnetization anisotropy, and manufacturingvariations.

There are several capacitor arrangements that give substantiallyimproved B₀ homogeneity under MAS compared to the prior art. For a 5 mmrotor diameter, the coilform outside diameter is typically 6.2 mm, whichwould permit a circle of 7 closely spaced capacitors of the typicalwidth--about 2.7 mm. But an even number of capacitors is required foracceptable B₁ homogeneity and ease of tuning, which limits the number ofcapacitors to six (three for each gap) with a little space between each.Such an arrangement results in greatly improved B₀ homogeneity comparedto the prior art, but capacitor lead routing connections are difficult.

FIG. 2 illustrates a preferred capacitor arrangement that requires onlyfour capacitors at each end, achieves even better resolution, and ismuch easier to assemble. The segmenting gap 21 at each end of eachsemi-coil is oriented in the circumferential direction between an innerarc 22 and an outer arc 23. Two ceramic chip capacitors 24, 25 arepositioned with respect to the transverse B₁ axis at approximate meanazimuthal angle a=35.3°, the complement of the standard magic angle, theangle with respect to B₀ at which the dipolar interaction vanishes. Atthis location, the mean B₀ field distortions in the sample regions nearthe capacitors are nearly zero for a spinner axis transverse to B₀. Withthe spinner axis at the magic angle and the sample spinning, the optimumangle is increased to about 45°, but angles between 25° and 65° arequite advantageous, as this contributes to cylindrical symmetry, whichaverages to zero under MAS.

The pattern shown in FIG. 2 achieves B₁ homogeneity comparable to thatof the conventional HT coil--though inferior to the SHT coil of FIG. 1.As with the prior HT coils, rf voltages near the central plane are zero,and the rf voltages are approximately balanced and equal at each end.The individual capacitors in each pair need not be equal for theseconditions to be met, although the sum of the capacitors in eachquadrant should be approximately equal. The effective axial field lengthis approximately equal to the axial distance between the gaps atopposite ends, but may be shifted a little if desired by adjusting theratio of the capacitors in each pair. As with FIG. 1, at low frequenciesthe capacitors at one end may be replaced by a short, thereby formingessentially a conventional slotted resonator. As with the prior art, thecapacitors at the coil would usually tune the coil slightly high, andleads would be attached across one of the capacitors for fine capacitivetuning and matching.

FIG. 3 is an approximate perspective view of an example of an MASspinner assembly utilizing magic angle capacitors with an HT cross coil.A copending application discloses other features of this particularspinner assembly in more detail. Briefly, the MAS rotor is inserted intothe opening 31 at one end. The spinner assembly may be oriented atvarious angles with respect to B₀ while air is supplied through an aircoupling/bearing 32 along the axis of orientation to the bearing airtube 33, which supplies bearing manifolds at each end. A similararrangement on the opposite side supplies the turbine drive gas.Capacitors 34, 35, 36, 37 are shown extending through a coversurrounding the HT coil and solenoid. Four more capacitors aresymmetrically located on the back side.

The capacitors in FIG. 3 appear spaced an additional radial distancefrom the coilform (its diameter may be judged from the size of theopening 31). Mounting the chip capacitors on (diamagnetic, copper orsilver) leads several millimeters in length to partially compensate theeffects of the paramagnetic capacitors is beneficial in furtherimproving B₀ homogeneity, as magic angle positioning only minimizes themost severe field distortions nearest the capacitors. Additionaldistance to the chips is quite beneficial in further improving B₀homogeneity. With copper leads, the total volume of copper required nearthe capacitor in the two leads is typically comparable to that of thechip capacitor, as their susceptibilities are comparable but of oppositesign. Silver leads, on the other hand, should be half as large. Itshould also be noted that the capacitors and coil patterns of the otherfigures are not indicative of typical relative scale for common high-Qrf capacitors and HT coils for a 5 mm spinner. The capacitors are atroughly two-thirds the scale of a coil for a 5 mm spinner.

FIG. 4 illustrates a possible method of achieving improved B₁homogeneity, Q, and efficiency by improving the uniformity of thecurrent density in a coil with magic angle capacitors. It comprisesessentially an inner segmented loop 41, 42 and an outer segmented loop43, 44 about the B₁ axis on each semicoil. The two loops in eachsemicoil are paralleled at one point in each azimuthal quadrant (in thiscase near the axial center) to suppress some unwanted modes. Otherwise,they are substantially magnetically coupled. The voltages acrosscapacitors 45, 46 are nearly in phase, in contrast to the birdcage, inwhich their phases would differ by 90° when there are four capacitors ateach end, for example. Since the inner loop has less inductance than theouter loop, the capacitors in the inner loop must be greater than thecapacitors in the outer loop, and homogeneity depends critically on thecapacitor ratios.

FIG. 5 illustrates a better method of improving B₁ homogeneity, Q, andefficiency by improving current distribution. Again each semicoilcomprises essentially two segmented loops, but in this case theyoverlap. That is, insulated cross-overs occur at two points in eachsemicoil--under capacitors 51, 52, for example--somewhat like a litzfoil coil as described in a copending application. This allows theinductances of the two loops to be equal, so all eight capacitors may beequal. Again, it is desirable to suppress as many unwanted modes aspossible, but the simple connections near the 90° plane as in theprevious coils are not as effective as another approach. If all loopsare electrically isolated and all have equal voltage and phaserelationships, tighter coupling (better suppression of false modes) maybe achieved by simply paralleling the capacitor pairs 52, 53 and 54, 55at one end on each semicoil (by eliminating the splits in the arcs atthis end, similar to FIG. 2 at one end) and then connecting the pairsfrom the two semicoils in parallel using a pair of jumpers with acrossover (for correct phasing) between capacitors 55 and 53. However,repeating this paralleling at the opposite end would short thedifferential flux between the intersecting loops and nullify theintended objective of improving current distributions.

Finally, FIG. 6 illustrates yet another way to improve performance bydriving an unsegmented low-inductance inner loop with half the totalouter-loop voltage by connecting it across the gap at one corner of theouter loops. For best homogeneity when all capacitors are equal, themean subtended angle of the inner loop will normally be about half themean subtended angle of the outer loops, but other ratios will beoptimum with other ratios of segmenting capacitors at opposite ends ofthe coil. This coil is similar to one disclosed in a copendingapplication where it was called an etched-half-turn fixed-frequency(EHTF) coil, as, unlike most other coils in that application, it is noteasily tuned over a wide range of frequencies. Tuning it withoutspoiling B₁ homogeneity requires simultaneous precise adjustment of fourpairs of capacitors. However, it has the advantage of being much easierto double tune (for example, for ¹ H and ² H lock) than alternative HTcoils.

What is claimed is:
 1. An NMR probe for use in an external field B₀ inthe z direction, said probe comprising:a cylindrical dielectric coilformnot aligned with the z axis, an rf coil wrapped around said coilformcapable of producing B₁ perpendicular to the plane defined by the axisof said coilform and the z axis, said coil further characterized ascomprising two substantially equivalent half-turn semicoils on oppositesides of said coilform, said half-turn semicoil characterized in that amajor fraction of the total current around the B₁ axis in said semicoilis interrupted by series capacitances of comparable magnitude at exactlytwo locations in its path around the B₁ axis, said series capacitanceseach comprising two effectively paralleled capacitors wherein the rfvoltage phase at said effectively paralleled tuning capacitors issubstantially equal at the intended frequency of operation, saidparalleled capacitors positioned individually at substantially equalaxial and radial positions near each end of said semicoil, said rf coilfurther characterized in that the mean subtended azimuthal angle betweensaid paralleled capacitors is greater than 50° and less than 100°. 2.The NMR probe of 1 further characterized in that each said capacitor ispositioned at approximate azimuthal angle of ±35° with respect to themean B₁ direction.
 3. The NMR probe of 1 further characterized asincluding diamagentic leads attached to said capacitors, said leadsfurther characterized as having mean magnetization comparable to thenegative of that of said capacitors.
 4. The NMR probe of 1 furthercharacterized in that said capacitors are radially spaced outward fromsaid coil by an amount greater than the minimum dimension of saidcapacitor.
 5. The NMR probe of 1 further characterized in that saidsemicoil comprises two separately tuned segmented loops including aninner segmented loop of lesser inductance and an outer segmented loop ofgreater inductance.
 6. The NMR probe of 1 further characterized in thatsaid semicoil comprises two separately tuned segmented loops of equalinductance and insulated crossovers at the connections to twocapacitors.
 7. The NMR probe of 1 further characterized as including anunsegmented inner loop connected across one of said tuning capacitors.